Metallized paper condensers



Dec. 3l, 1968 A|` |0 TOMAGO ET AL 3,419,770

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Invenlnrs @kia T0/hago Masayau Palmi ATTORNEYS Dec. 3l, 1968 Amo ToMAGo ET AL 3,419,770

METALLIZED PAPER CONDENSERS Sheet 2 of 4 Filed Feb. 8, 1967 Th/'ckneas af /nsu/af/hg paper (p) r f, a .mm

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ATTORN E YS Dec. 31, 1958 AKlO TQMAGO ET AL 3,419,770

METALLI ZED PAPER CONDENSERS Filed Feb. 8, 1967 Sheet 3 of 4- INVENTORS AK I O TOMAGO, MASAYASU FU KAI United States Patent O 3,419,770 METALLIZED PAPER CONDENSERS Akio Tomago, Ikeda-shi, and Masayasu Fukai, Suita-shi, Japan, assignors to Matsushita Electric Industrial C0., Ltd., Osaka, Japan, a corporation of Japan Continuation-impart of application Ser. No. 417,859, Dec. 14, 1964. This application Feb. 8, 1967, Ser. No. 622,863

2 Claims. (Cl. 317-258) ABSTRACT OF THE DISCLOSURE A metallized paper condenser formed from an insulating paper having electroconductive metal films vacuum deposited on both surfaces thereof. The metallized paper is wound with a second double-metallized paper, a singlemetallized paper, or a nonmetallized paper in superposition on the rst double-metallized paper.

This is a continuation-in-part of our application Ser. No. 417,859, tiled Dec. 14, 1964, now abandoned.

DISCLOSURE An object of the present invention is to provide a metallized paper condenser employing a thin paper strip having conductive metal layers on both sides of the paper (herein referred to as double-metallized paper) formed by vacuum `deposition in a single operation in one evaporation tank.

Conventional metallized paper has been heretofore constituted by a lacquer layer formed on insulating paper and a metal film vacuum deposited thereon for improving the electrical characteristics of condensers constructed with said metallized paper, for example, for elevating the insulating resistance, for reducing dielectric loss, and for elevating the breakdown voltage thereof. However, according to the present invention, it has been found that condensers having the characteristics described below are obtainable by selecting the space factor of cellulose of insulating paper properly, metallizing both surfaces of the insulating paper so that the size of microcrystal grains constituting the metal iilm deposited thereon will be within a certain proper range, and suppressing the number of conductive particles contained in said insulating paper below a certain proper value. Thus, metallized paper having directly vacuum deposited metal layers on both sides thereof may be employed without a lacquer layer. That is,

(l) The metal film is directly deposited on both surfaces of the insulating paper. Therefore, no air spaces exist between the metal film and insulating paper. In consequence, the metallized paper has a high electrostatic capacity per unit area and, as a result thereof, by using the metallized paper, condensers may be compact, light in weight, and economically produced.

(2) The surface of the insulating paper is coarse with recesses and projections and therefore the surface area thereof is large. In consequence the electrode area thereof becomes large when the metal is directly deposited on the papers surface without a lacquer layer, and as a result thereof, the electrostatic capacity per unit area thereof increases. Such effect appears more significantly as the insulating paper becomes thinner, and as the result thereof, by using the metallized paper, condensers can be compact, light in weight, and economically produced.

(3) The cost per unit area of insulating paper, when metallized, doubles that of the paper alone. This rise in the cost is not due entirely to the cost of material (the cost of the metal film deposited, for instance), but the major part of it is due to the cost of the metallization process. Consequently, if both surfaces of the insulating ICC paper are metallized in a single vacuum evaporation tank by means of a device wherein both surfaces are simultaneously vacuum deposited with the metal, the cost per unit area of double-metallized paper will be substantially the same as the cost of single-metallized paper, i.e., paper having only one side surface thereof metallized. Therefore, condensers constructed with double-metallized paper and unmetallized paper, which is low in cost, can be produced for about 25 percent less than condensers constructed with only single-metallized paper.

(4) According to the present invention, in which both surfaces of the insulating paper are simultaneously metallized in the same vacuum evaporation tank, the production efliciency of the double-metallized paper is doubled in comparison with that of conventional single-metallized paper. As a result thereof, the production efficiency of the condensers is high and thus the condensers can be produced economically.

(5) When both surfaces of insulating paper have metal films thereon, without lacquer therebetween, the metal films are deposited intimately on both surfaces of insulating paper and the effective insulation thickness between the metal films is smaller than the case wherein singlemetallized paper is used. Since the cost of insulating paper rises proportionally as the thickness decreases, low voltage condensers may bed easily designed and economically produced with double-metallized paper.

(6) The frequency, namely the number of occurrences per unit area of self-healing in a condenser having doublemetallized insulating paper without a lacquer layer is somewhat higher than for a condenser using conventional single-metallized insulating paper. The self-healing of a condenser is a phenomenon in which, when a high voltage is applied between the electrodes of the condenser, an electrical weak point of a dielectric is broken down and electricity is discharged therethrough. Under such a circumstance, a part of the electrodes, i.e., the metal iilms are burnt out and the insulation between the electrodes is recovered. The electrical energy in self-healing is mostly consumed in arcing heat and by the decomposition of the dielectric with a very small part thereof being consumed in the burning out of a part of the metal films.

When the dielectric is decomposed, free carbon and hydrogen gas are generated. These gases induce a further self-healing and lower the characteristics of the condenser. In order to reduce the bad influence of such selfhealing of a condenser, double-metallized insulating paper as described below has been developed.

(a) In the course of supercalendering insulating paper,

by having the pressure on the insulating paper controlled, the space factor of cellulose of the insulation paper suitable for double-metallized paper has been determined.

(.b) Both surfaces of insulating paper have been vacuum deposited 4with a metal so that the size of the microcrystal metal grains will be within a properly predetermined range.

(c) Only insulating paper having the number of conductive particles contained therein Ibelow a certain predetermined fvalue has been used for double-metallized paper.

The insulating paper for double-metallized paper is made from wood pulp as is the common insulating paper for the condensers. The pulp employed for double metallized paper averages about 30-50 am. in its cellulose fiber length. When paper is made of such pulp, the thin iii-bers are beaten so as they intermingled suiciently and closely. The finely beaten thin films are separated in skimming water and so adjusted that the consistency of the sl-urry 1s 0.5-3% (this value differs according to the thickness of paper) and is forwarded to a skimming machine at a constant flowing quantity. In this case the temperature of the skimming water is about -30" C. The paper reeled at the final stage of the skimming machine is low in density, about 0.5-0.8 g./cc. Consequently, in order to raise the density of the final paper, the density of the paperlike pulp sheet is controlled in the course of supercalendering. When this supercalendering is performed, the water content of the paper is -25% and its temperature 1GO-150 C. Thus the insulating paper for the dou- |ble-metallized paper, which has been beaten thoroughly and had high pressure applied during supercalendering, has a high density.

'Since the characteristics of a condenser, constructed of double-metallized insulating paper, are influenced by the self-healing, the generation of the self-healing has to be suppressed as much as possible. For that purpose, the volumetric proportion occupied by the cellulose constituting the insulating paper is to be made as large as possible and the air space portion of the insulating paper is made as small as possible. There are various kinds of cellulose for -use in insulating paper such as kraft cellulose, acetyl cellulose, ethyl cellulose, etc. Since these celluloses are different in specific weight, even if the amount of cellulose contained in a unit volume of the insulation paper is the same, the density of the insulating paper will be different for each cellulose. Consequently, according to the present invention, the quantity of cellulose contained in a unit volume of insulating paper is not expressed by density but is expressed by the volumetric proportion occupied by the cellulose therein, and it is represented by the space factor of cellulose.

The space factor of cellulose is measured by a method described below. This measurement gives the density (apparent specific gravity) of the insulating paper. From this value, the space factor of cellulose can be determined by calculation from the following formula, using the value of the specific gravity of the cellulose which may be measured in other ways.

where S=space factor b=density in grams per cubic centimeter (g./cm.3) G=specic gravity of cellulose Assuming that the type of insulating paper is kraft paper whose cellulose has a specific gravity of 1.53, as is well known, and that the density determined for the various types of insulating paper are 0.8 g./cm.3, 0.7 g./cm.3, and 0.54 g./cm.3, respectively, the space factor of cellulose in the respective insulating paper is as follows:

More than 52% for the insulating paper for the doublemetallized paper,

More than 46% for the insulating paper for singlemetallized paper to be used along with the double-metallized paper, and

More than for the insulating paper for the nonmetallized paper to be used along with the double-metallized paper.

The size of the `metallic microcrystal grains deposited on both surfaces of the insulating paper, in order that the double-metallized paper has satisfactory electrical characteristics, should be 1,500 A. to 100 A. As for the manufacturing process of the double-metallized paper, the explanation has *been given in paragraph (C) below. The conditions to control the size of the micrograin crystal are described above.

It has also been found that, when self-healing is generated, in order to reduce the decomposition of the dielectric and to decrease the electrical energy discharged, the size of microcrystal grains of metal constituting the metal deposited films has to be within a proper range. The deposited metal lm is constituted by the aggregation of microcrystal grains. In'order to control the size of the grains properly, the temperature of the metal vapor source and the substrate to "be metallized and the amount of deposition of the metal employed as a nucleus for crystallization are controlled. A metal nucleus of (Q01-0.1) 103 mg./cm.2 is necessary for double-metallized insulating paper.

The characteristics of a condenser made from the double-metallized insulating paper according to the present invention are as above described, and the various conditions that should be provided in such a condenser are now described with reference to the accompanying drawings in which:

FIGS. 1 and 2 are diagrammatic sectional views of two forms of conventional metallized paper condensers;

FIGS. 3, 4 and 5 are diagrammatic sectional views of three forms of metallized paper condensers embodying the present invention;

FIG. 6a is a diagrammatic sectional view of the doublemetallized paper before cutting;

FIG. 6b is a sectional view of the double-metallized paper after having been cut;

FIG. 7 shows a perspective view of one of the methods for winding the double-metallized paper into a roll-type condenser unit;

FIGS. 8a, 8b, 9a, and 9b show condenser units ern.- bodying the invention;

FIG. 10 is a graphic representation of the relation between the thickness of insulating paper and the capacity per unit area; and

FIG. 11 is a graphic representation of the rel-ation between thickness of insulating paper and the ratio of selfhealing frequency.

Referring to FIG. 1, an `insulating paper strip 1 has a lacquer layer 2 of 1.0 to 1.5 microns thickness applied on one face of the paper which is presensitized with metal nuclei. A conductive metal lm 3, such as zinc, is then formed `thereon by vacuum deposition. T wo such metallized papers, in superposition, are wound together to constitute a condenser unit. However, the capacity per unit area obtained by 'use of metallized paper, as shown in FIG. l, is relatively small. Also, in low voltage condensers, reasonable design is difficult since there is a lower limit to the thickness of the paper. Moreover, such a metallized paper with a lacquer layer is apt to be high in production costs due to lacquering.

In FIG. 2 the lacquer layer has been omitted and a conductive metal 3 is deposited directly on one side of an insulating paper strip 1. Two such paper strips are superposed with a layer of nonmetallized insulating paper 4 superposed therebetween, and the laminated strips are wound together to form a condenser unit. The paper strips 4 are used to increase the rating voltage of the condenser unit resulting in the capacity per Iunit area being small. By virtue of the interposition of the nonrnetallized paper strip 4, there is a substantial lack of self-healing action in this prior art condenser. Consequently, it is of no use to give critical requirements on the characteristics of the insulating paper 1 and the deposited metal 3.

Referring to FIGS. 3 to 5 showing the present invention, insulating paper 1 has a conductive metal 3 deposited on both sides of the paper strip 1, which may be presensitized with metal nuclei if necessary. At least one such metallized paper is employed to form a condenser by winding up the same with superposition thereon of a nonmetallized insulating paper 4 (FIG. 3) or an insulating paper 1 with a conductive metal 3 directly deposited onone of two opposite faces of the paper (see FIG. 4) hereinafter referred to as a single-metallized paper. Alternately, as shown in FIG. 5, two strips of the double-metallized paper may be superposed together and wound up to constitute a condenser.

The characteristics of the condenser, constructed of double-metallized insulating paper as described above, are high electrostatic capacity per unit area of the doublemetallized insulating paper, the condenser is compact,

light, and low in cost, the working efciency of vacuumdeposited metal is high, the condenser can be produced economically, and -the design of the condenser for low voltage is simplified. On the other hand, however, it has been found that its frequency of self-healing is slightly higher and has an influence on the characteristics of the condenser. According tothe present invention, the former advantage is kept while the latter disadvantage has been obviated by proper selection of the requirements for the space factor of cellulose in the insulating paper, the microcrystal grain size of the deposited metal films on the paper surface, etc. Pnior to the present invention, it was not known what various conditions were necessary for the double-metallized insulating paper. That is, in conventional |metallized insulating paper condensers, selfhealing as above mentioned has no occurred and the strict requirements for double-metallized insulating paper have not been necessary. When double-metallized insulating paper is employed, condensers of good electrical characteristics are not obtainable unless the space factor of cellulose of the insulating paper land the size of the microcrystal grains constituting the metal films thereof is a proper value.

In conventional single-metallized insulating paper, the thickness of the metal film thereon has been in the range of 0.03 to 0.08p and the surface resistivity thereof has been in the range of 0.75 to 1.751Q/cm-2. However, with double-metallized insulating paper having metal films of such values, condensers of good characteristics cannot be manufactured.

As imentioned above, in double-metallized insulating paper condensers, according to the present invention, various features provided therein are utilized. In order to suppress the defects thereof to a degree which is not probliematical in practice, various points, such as the number of conductive particles contained in the paper and the insulation coordination between -the double-metallized insulating paper and the unmetallized paper, besides the conditons for the insulating paper and the metal films essential for double-metallized paper, have been researched and studied. This has resulted in the inventive condensers 'which have good electrical characteristics and are compact, light weight, rand economical to produce.

Various factors for accomplishing the objects of the present invention will now be described in full. Items A to E in the following describe the requirements or conditions essential for double-metallized insulating paper condensers according to the present invention.

(A) Space factor of cellulose for insulating paper The volume of insulating paper is calculated by determining the thickness of one sheet of insulating paper by means of measuring, with a micrometer, the total thickness of several layers of insulating sheets superposed together, after having kept the sheets for a long period of timeat the standard conditions of 20 C.| 5 C. in temperature and 65 %,i2% in relative humidity. The weight of such paper sheets are then measured after drying them in air at a temperautre of l05i3 C. The space factor of cellulose of the thin paper strips have been calculated as follows:

Double-metallized paper over 52% Single-metallized paper over 46% Nonrnetallized paper over 35% (B) Conductive particles The number of conductive particles contained in the insulating paper should be as low as possible. According to the current papermaking techniques, it is impossible to manufacture paper not containing any of such particles. With regard to metallized papers with metal films directly deposited on the surfaces thereof, if the paper is relatively thin, the metal film is liable to contact conductive particles contained in the paper, particularly in the case of doubleanetallized paper when a greater probability may be expected. Consequently, Imany more electrically weak spots would inevitably exist in such metallized paper so that it is necessary to require a strictly determined upper allowable limit for conductive particles contained in the paper. Table 1 gives the allowable upper limits of conductive particles contained in double-metallized paper, single-metallized paper, and nonrnetallized paper employed according to the present invention.

TABLE L ALLOWABLE MAXIMUM NUMBER OF CONDUC- TIVE PARTICLES PER UNIT SQUARE METER Thickness of Double Single Nonmetallized Paper (n) Metallized Metallized (C) Manufacture of double-metallized paper Insulating paper contains several percents of Water under normal conditions. The water content of chemically 'modified paper, such as cyanoethylated paper is relatively low, but before metallizing such paper, it must be subjected to vacuum drying under heat in order to sufficiently remove both water and gas therefrom. Thus, the water content of insulating paper before vacuum deposition should be less than 0.1%. Insulating paper that has been dried under heat in a vacuum prior to the vacuum deposition is double metallized by evaporation within a single tank and metallized paper, such as shown in FIGS. 6a and 6b, is obtained. FIG. 6a shows a cross section of a paper strip 1 that has been metallized and carries vapor deposited metal films 3 on both surfaces with insulating margins between successive metal films 3', each margin having a width of 4 to 6` mm. This paper strip is cut along the dotted lines into a number of elements of metallized paper as shown in FIG. 6b. In each element the margin portion is 2 to 3 mm. in Nvidth.

(D) Vacuum deposited metal film VIn double-metallized paper condensers, self-healing is likely to take place. The aftereffect of self healing should be as small as possible and the condenser characteristics should be stable. The condition for satisfying such requirements is that the average diameter of the microcrystal grains of the deposited metal iilrn should be 100 A. to 1500i A.

(E) Insulation'coordination between respective paper strips of a condenser A doublemetallized paper condenser, according to the present invention, is manufacured as shown in FIGS. 7, 8a, 8b, 9a, and 9b by use of double-metallized and/or nonmetallized paper strips as shown in FIGS. 3, 4 and 5. In FIG. 3 the nonmetallized paper strip 4 should have a width narrower by 0.5-2.0 mm. than the doublemetallized strip 1 in order to have a large sectional area of the metal film exposed beside the takeup roller 11 (see FIG. 7) for providing ample electrical contact with external lead wires. The thickness of nonmetallized paper may be equal to or smaller than that of double-metallized paper. When the former is thinner than the latter, the difference therebetween is determined in accordance with the difference in densities of both paper strips, as shown in Table 2.

TABLE 2 density of double metallized paper. 1.00 0.92 0.84 0.76 0.68 0.60 Thickness of double-metallized paper minus thickness of singlemetallized paper (p) 5 3 2 1 0 0 When the double-metallized paper is very thin, say less than l microns, the thickness of the nonmetallized paper need not be as thin as is necessary to follow the condition given in the above table if the papermaking technique or cost is prohibitive.

The double-metallized paper strip and nonmetallized paper strip shown in FIG. 3 are wound up together in a manner as shown in FIG. 7. In this figure, a doublernetallized paper strip 5 is fed out from a roll 6 and a nonmetallized paper strip 7 is fed out from a roll 8, both strips being guided by rollers 9 and 10, respectively, and wound up in superposition on takeup roller 11. FIG. 8a is a perspective view of a condenser roll thus formed. This roll is then pressed into a flat-type condenser unit, as shown in FIG. 8b. The condenser roll-type unit, shown in FIG. 9a, is used as a condenser unit as it stands, as sho-wn in FIG. 9b. The condenser elements shown in FIG. 8b and FIG. 9b, respectively, are provided with terminal metal layers .12 and lead wires 13 at both ends. The terminal metal layer 12 is formed by spraying a molten form of a metal lhaving a low melting point or by painting the end of the element with an electroconductive material. The condenser unit as is above described is impregnated and sealed with a liquid or wax after vacuum drying. A similar process may be adopted for making condenser elements by use of the strip assemblies as shown in FIGS. 4 and 5.

Special features of double-metallized paper condensers, according to the present invention, manufacturing conditions thereof and their functional effects will be described by way of practical examples from paragraphs I to IV.

I. Space factor of cellulose in insulating paper The metal film deposited on the surface of insulating paper is formed upon the rough surface of the paper. Consequently, the metal penetrates deeply into the paper at points where little cellulose is present, by reason of uneven distribution of the cellulose or a low space factor of cellulose, resulting in electrically weak spots. In order to eliminate such faults, the space factor of cellulose should be uniform throughout the paper and be of proper value. The best feature of the dou'ble-metallized paper is that the capacity per unit area increases. This is due to the increased electrode area as well as the close adhesion of the metal lms to the dielectric material. FIG. shows the ratio of capacity per unit area of double-metallized paper in comparison to conventional single-metallized paper and other paper. yIn FIG. 10, the electrostatic capacities per unit area, derived from the electrostatic capacities of condensers formed from conventional metallized paper of various thicknesses, are made a reference value of 100. The electrostatic capacities per unit area of double-metallized paper and that of unmetallized paper are each compared therewith. As is clearly seen from FIG. 10, the capacity per unit area of the double-metallized paper is particularly large and a condenser formed thereby may `be far 'more compact and light in comparison to that formed by any of the conventional metallized papers.

In double-metallized paper condensers, a lower space factor of cellulose results in a large capacity per unit area. But too low a space factor of cellulose results in too many electrical weak spots, as hereinbefore described, and the condenser characteristics cannot be good because of too great lan occurrence of self healing. With regard to single-metallized papers and nonmetallized papers employed in combination with double-metallized papers, according to the present invention, a low space factor of cellulose results in unstability of the condenser characteristics, especially in life tests. It was found that the low space factor of cellulose in any of the insulating papers, either metallized or nonmetallized, results in a high frequency of self-healing action thus producing poor quality products.

EXAMPLE 1 Zinc (Zn) was vacuum deposited onto both surfaces of insulating paper of 14p thickness. The metallized paper was rolled into condenser elements, with nonmetallized,

papers of definite thickness and space factor of cellulose in superposition therewith. The space factor of cellulose of the double-metallized papers were 44%, 48%, 52%, 65.3% and 81.7%, respectively. The condensers made with the paper of 44% and 48% space factor of cellulose had poor and unstable characteristics. The condenser having a 44% space factor, for example, had `an insulation resistance of 560 ohm-farads and a dielectric percent loss of 0.48%. After a Z500-hour life test, the insulation resistance `decreased to ohm-farads and the dielectric percent loss increased to 1.8%. In comparison to the above, the condensers of a space factor of cellulose above 52% were stable in the initial characterisitcs as Well as in the life characteristics. The condenser of 65.3% space factor for example had an insulation resistance of 5200 ohm-farads and a dielectric percent loss of 0.48%. After a Z500-hour life test, the insulation resistance was 6000 ohm-farads and the dielectric loss was 0.50%. The condenser having a 52% space factor also showed similar results. It has Ibeen found that the higher the space factor of cellulose, the more stable the various characteristics.

EXAMPLE 2 Double-metallized paper, similar to that used in Example I, was employed together with single-metallized paper Iand double-metallized paper of definite thickness and space factors of cellulose, in combination, for making condensers. Similar results to those of Example I were obtained. That is to say, condensers having a greater than 52% space factor of cellulose in double-metallized paper had good characteristics.

EXAMPLE 3 Condensers were made with double-metallizedv papers of definite thickness and cellulose space factor in combination with single-metallized papers of 12u thickness and a space factor of cellulose of 34.7%, 42%, 46%, 55.5% and 71.9%, respectively. The characteristics before and after Z500-hour life tests were measured on each condenser and it was found that those condensers having a space factor below 46% had unstable characteristics. Before the life test, the insulation resistance was about 3500 ohm-farads and the loss was 0.52%. After the life test the insulation resistance decreased to 1900 ohmfarads and the loss increased to 0.72%. In addition, the outer casing expanded about 7% because of the high pressure gas produced by too much self-healing. Those condensers of higher than 46% cellulose space factor had stable first period characteristics before and after the Z500-hour life test, and the higher the factor the better and more stable the characteristics.

EXAMPLE 4 Double-metallized papers of definite thickness and space factor of cellulose of 52%, 65.3% and 81.7%, respectively, were employed for making condensers in combination with nonmetallized papers of 10p thickness and of a space factor 'of cellulose of 55% and 67%, respectively, of those of the double-metallized paper strips. Their initial characteristics as well as life characteristics after a Z500-hour life test were measured and the results were that the condensers composed of double-metallized paper of 52% space factor and nonmetallized paper of 55% of the former factor showed very unstable initial characteristics and particularly after the life tests the dielectric loss increased markedly. The space factor of nonmetallized paper in the above case is about 29% (52% x 55%). When the double-metallized paper had 65.3% factor and nonmetallized paper had a factor of about 36% (55% x 65.3%), the initial characteristics and life test characteristics were good.

II. Conductive particles Metallized insulating paper with metal lms directly deposited onto the surfaces thereof is apt to have more electrically defective spots because it has more conductive particles to contact the metal films in comparison to those having intermediate lacquer layers. Particularly for thin papers such a probability of having electrically de- Afective spots is much greater. However, since metallized papers have self-healing action, contact between conductive particles and the metal films to a certain extent does not materially affect the condenser characteristics. But with regard to double-metallized papers, it was found that the frequency of self-healing action was higher so that it is necessary to limit the content of conductive particles in order to obtain condensers of good characteristics. Therefore, the inherent content of conductive particles in the insulating paper employed should be lower than conventional insulating paper. It was found that the influence upon condenser characteristics of the number of conductive particles contained in paper was particularly remarkable in the case of double-metallized paper.

The limits of content of conductive particles were investigated in the hereinabove described Examples l to 4 to determine the maximum limit allowable, the results being shown in the hereinbefore presented Table 1. By use of insulating paper strips having a conductive particle content lower than those given in Table 1, doublemetallized paper condensers can be obtained with a large capacity per unit area, are compact and light in construction, and have good characteristics.

III. Deposited metal film As has been clarified hereinabove, a conventional singlemetallized paper comprises an insulating paper and a metal film deposited on one side thereof with a lacquer layer interposed therebetween. A suitable prebreakdown treatment is effected before use of the condenser. When a single sheet of insulating paper is used between two electrodes and voltage is applied thereto, the opposing electrodes may be electrically short circuited since minute conductive grains exist in the sheet. Therefore, it is necessary to apply a high DC voltage to the insulating paper between the electrodes to have a self healing caused therein and thereby to remove the short-circuited portion thereof in order to have the insulation recovered. This treatment is called prebreakdown treatment. There is substantially no self-healing action during the use of the condenser. Consequently, self healing has substantially no influence on the useful life of the condenser. Among the factors affecting the useful life of such a condenser, oxidation of the deposited metal is remarkably effective. Particularly in insulating paper having a lacquer layer, the difficulty of removing gas and dehydrating varies according to the kind of lacquer and solvent used. The metal deposited onto the paper in an atom-like or microscopic state is likely to be oxidized.

Consequently, it was found, in the conventional metallized paper condensers, that the decrease of life characteristics was remarkable when the microcrystal grains of the deposited metal film are small. For example, if the average diameter of grains is 1800 A., the rate of decrease of insulating resistance in life tests was 85-100%, while, if the average diameter is 3200 A., the rate of deterioration was 35-45%.

The falling rate of insulation resistance is represented as follows:

ERO-RLl where D=the falling rate of insulation resistance R=the initial value of insulation resistance Rl=the insulation resistance after life tests for t hours method. However, with regard to the condenser of the present invention, employing double-metallized paper, the decrease of useful life is wholly different from that of a conventional single-metallized paper condenser, and it has been found that the specific double-metallized paper should be employed.

Since there were a few more electrically defective spots in double-metallized paper than in the conventional paper, it was found that self healing of condensers composed of the double-metallized paper took place at a lower rate of frequency during running, even after a suitable prebreakdown treatment. Thus, in such a case, it was found that the life deterioration is largely affected by self-healing action during use of the condenser. It was found that the grain sizes of the deposited metal lm had to be made small in the double-metallized paper, there being no interposing lacquer layer, in order to make the deposited film adhere intimately to the relatively rough paper surfaces. The deposited films are in close contact with the paper, and the beam energy of the particles themselves, during the vacuum deposition, is small. Therefore, proper self-healing action is effected by little energy resulting in a small detrimental effect on the dielectrics.

Contrary to the above, when grain sizes are large, the beam energy of the particles themselves is large so that local depth of the particles into markedly uneven insulating paper surfaces are naturally deep resulting in an extremely short insulating distance between electrodes. On the other hand, metal may adhere to the paper surface in Ia bridge-like manner at extremely uneven portions of paper surface. In an extreme case, an air space between the metal film and insulating paper might be formed. For the above reason, the mutually facing electrodes of the condenser are formed with more uneven spacing than the roughness of paper surfaces and the self-healing action consumes large energy resulting in a detrimental influence on the dielectrics. FIGS. 12 and 13 show the relation of the average diameter of crystal grains of deposited film, the insulation resistance of the double metallized paper, Vand tan As it is clear from the drawings that the electric characteristics of the condenser will be poor with either larger or smaller crystal grains, and show that good electric characteristics are obtained when the size of the crystal grains is within a certain proper range.

The range of grain sizes of such deposited metal films is classified as below 1500 A. and above 100 A. For example, when the average diameter of grains is 1800 A., as measured by the Timmer method, the rate of decrease of the insulating resistance in the life tests is -100%, while when the average diameter is 600 A., the rate of decrease is 10%.

However, in the present condenser, employing doublemetallized paper, the mechanisms of deterioration are wholly different from conventional condensers having a single-metallized paper with interposed lacquer layer. Condensers of good characteristics are provided by employing double-metallized paper Without a lacquer layer and containing microcrystal grains of a size within the range of -1500 A., which is outside the range of grain size used in conventional metallized paper condensers.

Metals suitable for use in the present invention may include zinc, cadmium, aluminum, magnesium, tin, lead, etc., among which zinc and cadmium are most suitable.

IV. Formation of a condenser Combinations according to the present invention of double-metallized, single-metallized, and unmetallized paper are shown in FIGS. 3, 4, and 5. In the case of FIG. 5, in which the condenser is formed with doublemetallized paper only, the two strips of paper are equal in thickness. But in the case of FIGS. 3 and 4, the thickness of the double-metallized paper may be different from that of the nonor single-metallized paper for effecting a reasonable design.

FIG. 11 shows results of comparison of the self healing occurrence per square centimeter of the respective insulating papers in the constructions of FIGS. 3 and 4. In FIG. l1, the results are shown for unity ratios of the densities of nonmetallized paper to double-metallized paper and of single-metallized paper to dou-ble-metallized paper. Thus, when the densities of respective papers are equal, the frequency of self healing in the respective papers becomes equal when:

The thickness of double-metallized paper minus that of nonmetallized paper equals 8p,

The thickness of double-metallized paper minus that of single-metallized paper equals a.

In the case when the ratio of densities of the paper strips is not equal to each other, Table 2 may be utilized for obtaining insulation coordination between the double-metallized paper and the non-or single-metallized paper to have equal frequency of self-healing action, whereby condensers are provided with reasonably stable characteristics and enlarged capacity per unit area.

As has been clearly set forth from the foregoing examples of the present invention, the new condensers have good characteristics of life and stability together with large capacity per unit area and result in comp-act, light, and economical double-metallized paper condensers. In addition, by virtue of double-metallized papers employed in the present invention, efciency in the manufacture is markedly increased and the availability is extremely large.

The invention may Ibe embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore to be embraced herein.

What is claimed is:

1. A metallized paper condenser comprising at least a double-metallized paper strip, layers of electroconductive crystalline metal grains directly vacuum deposited on both surfaces of said paper strip, a nonmetallized insulating paper strip in superposition with said double-metallized paper strip, said nonmetallized insulating paper strip having a space factor of cellulose higher than 35 percent, the insulating paper having a space factor of cellulose higher than 52 percent and the average diameter of the crystalline metal grains being in a range of from A. to 1500 A.

2. A metallized paper condenser comprising at least a double-metallized insulating paper strip, layers of electroconductive crystalline metal grains directly vacuum deposited on both surfaces of said paper, a nonmetallized insulating paper strip in superposition with said doublemetallized paper strip, said insulating paper forming the base of said double-metallized paper having a space factor of cellulose higher than 52 percent, the average diameter of the crystalline metal grains being in the range from 100 A. to 1500 A. and said nonmetallized insulating paper being thinner than said double-metallized paper.

References Cited UNITED STATES PATENTS 971,667 10/1910 Dean 317-260 FOREIGN PATENTS 722,636 1/ 1955 Great Britain.

OTHER REFERENCES Birks, I. B.: Modern Dielectric Materials, London, Heywood Co., 1960, pp. 30 and 31.

LEWIS H. MYERS, Primary Examiner.

ELLIOT A. GOLDBERG, Assistant Examiner.

U.S. C1. X.R. 

